The present invention applies to a mass airflow measuring apparatus that measures the mass airflow through the suction air path of an internal combustion engine, particularly to an heating-resistor type mass airflow measuring apparatus that measures the mass airflow using an heating resistor.
In a conventional heating-resistor type mass airflow measuring apparatus, the current through the heating resistor is so controlled that the heating resistor temperature is higher by a specified temperature than that of the temperature-sensing resistor that detects the suction air temperature, and the suction mass airflow is measured in terms of the current through the heating resistor.
The conventional heating-resistor type mass airflow measuring apparatus is found to involve a problem that, when the engine has stopped and the airflow through the suction system has ceases, gaseous material flows by diffusion and evaporation out of the engine and its apparatuses installed in the suction system in the downstream of the heating-resistor type mass airflow measuring apparatus and reaches the installation point of the heating-resistor type mass airflow measuring apparatus and adheres onto the heating resistor, a mass airflow detecting element. In particular, since volatile gas such as oil vapor evaporates and diffuses when the temperature of the engine or its apparatuses installed in the suction system is higher than the vaporization temperature of the gas after the engine stoppage, and flows backward to the installation point of the heating-resistor type mass airflow measuring apparatus, it adheres onto the heating resistor exposed to the suction air. The adhesion of the gas results in the degradation of the measuring accuracy of the heating-resistor type mass airflow measuring apparatus because of corrosion and/or contamination of the heating resistor. In addition, the adhesion may result in accelerated adhesion of foreign substance such as dust onto the heating resistor. As a result, there arises a problem that the measuring accuracy of the mass airflow measuring apparatus degrades.
As an art relating to the present invention, there is disclosed a mass airflow measuring apparatus, for example, in a Japanese Application Patent Laid-Open Publication No. 3146850, where the apparatus is kept electrified continuously even after the engine stoppage so as to lessen the degradation of the measuring accuracy of the mass airflow measuring apparatus just after the engine start. This, however, continues electrification of the mass airflow measuring apparatus throughout the engine stoppage and the object of the electrification is to lessen the degradation of the measuring accuracy just after the engine start.